Stop migrating Cursor sandbox settings (#39325)

## What changed

- Ignore `.cursor/sandbox.json` during external agent config migration.
- Continue importing supported settings from `.cursor/cli-config.json`.

## Testing

- Update app-server migration coverage to verify that environment variables from `cli-config.json` are imported.

GitOrigin-RevId: 6094f1cb6f698392d58056f826da21c1f1389a1b

codex-rs/app-server/tests/suite/v2/external_agent_config.rs

@@ -338,7 +338,10 @@ async fn external_agent_config_migration_source_drives_detect_and_import() -> Re
     let codex_home = TempDir::new()?;
     let source_home = secondary_external_agent_home(codex_home.path());
     std::fs::create_dir_all(&source_home)?;
-    std::fs::write(source_home.join("sandbox.json"), r#"{"type":"read_only"}"#)?;
+    std::fs::write(
+        source_home.join("cli-config.json"),
+        r#"{"env":{"SOURCE":"secondary"}}"#,
+    )?;
     let home_dir = codex_home.path().display().to_string();
     let mut mcp = TestAppServer::builder()
         .with_codex_home(codex_home.path())
@@ -387,7 +390,7 @@ async fn external_agent_config_migration_source_drives_detect_and_import() -> Re
     assert_eq!(completed.item_type_results[0].failures, Vec::new());
     assert!(
         std::fs::read_to_string(codex_home.path().join("config.toml"))?
-            .contains("sandbox_mode = \"read-only\"")
+            .contains("SOURCE = \"secondary\"")
     );
 
     Ok(())